BERLIN/MOSCOW (Reuters) - Ukrainian President Petro Poroshenko accused Russia’s Vladimir Putin on Thursday of wanting to annex his entire country and called for NATO to deploy warships to a sea shared by the two nations.

Poroshenko’s comments to German media were part of a concerted push by Kiev to gain Western support for more sanctions against Moscow, securing tangible new Western military help, and rallying opposition to a Russian gas pipeline that threatens to deprive Ukraine of important transit revenues.

His Western allies have so far not offered to provide any of these things, despite his warnings of a possible Russian invasion after Moscow seized three Ukrainian naval ships and their crews on Sunday near Crimea.

Moscow and Kiev blame each other for the incident, which took place in the Black Sea near Crimea, the Ukrainian region annexed by Russia in 2014.

“Putin wants the old Russian empire back,” he said. “Crimea, Donbass, the whole country. As Russian tsar, as he sees himself, his empire cannot function without Ukraine. He sees us as his colony.”

Putin has accused Poroshenko of manufacturing the crisis to boost his flagging ratings ahead of next year’s elections in Ukraine.

Volodymyr Omelyan, Ukraine’s infrastructure minister, said on Thursday Russia had imposed a de facto blockade on two Ukrainian ports on the Sea of Azov by barring ships from leaving and entering the sea via the Kerch Strait.

The Kremlin denied it was restricting shipping, saying it had not heard of any problems.

WARSHIPS

Poroshenko told Bild he also wanted NATO to deploy warships to the Sea of Azov. Ukraine is not a member of the U.S.-led military alliance.

NATO, which has condemned Russia’s seizure of the Ukrainian ships, said that the alliance’s ships routinely patrolled and exercised in the Black Sea, that allies conducted regular air policing and reconnaissance flights there, and that the alliance had stepped up its activities since Moscow annexed Crimea.

“Three NATO members - Bulgaria, Romania and Turkey - are littoral states with their own national capabilities deployed in the Black Sea region,” NATO spokeswoman Oana Lungescu said.

“So there is already a lot of NATO in the Black Sea, and we will continue to assess our presence in the region.”

In a sign of support for Ukraine, U.S. President Donald Trump cancelled a planned meeting with Putin at the Group of 20 summit in Buenos Aires.

“Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina with President Vladimir Putin,” Trump tweeted.

German Chancellor Angela Merkel said she thought there was no military solution to the crisis.

The Kremlin said Poroshenko’s request for naval support appeared to be aimed at increasing tensions in the area.

The prospect of NATO warships heading to the Sea of Azov which is separated from the Black Sea by the Kerch Strait seems remote as it is shallow, access to it is controlled by Russia, and the Kremlin would likely view any attempt by the Atlantic alliance to deploy there as a hostile act.

Poroshenko, who has temporarily imposed martial law in parts of Ukraine over the crisis, also tweeted on Thursday that he would impose unspecified restrictions on Russian citizens in his country.

Ukraine’s border service said it would only allow Ukrainian citizens to travel to Crimea via its land border with the annexed territory, while the head of the Ukrainian navy said Kiev would try to get Turkey to close the Bosphorus Strait to Russian ships.

There were further signs that Russia was pressing ahead with its plans to fortify Crimea and turn it into what Kremlin-backed media have called a fortress.

MISSILE SYSTEMS

Russia on Thursday deployed a new battalion of advanced S-400 surface-to-air missile systems in Crimea, its fourth such battalion, TASS news agency cited a spokesman for Russia’s Black Sea Fleet as saying.

Citing a Crimean security source, Interfax news agency also reported Russian plans to build a new missile early-warning radar station in Crimea next year that would be able to track ballistic and cruise missiles from a long distance.

Russia was also working on a new technical system to allow it to better track shipping around the peninsula in order to protect its maritime borders, Interfax said.

The United States and the EU have imposed sanctions on Russia over its conduct towards Ukraine since 2014, when Moscow annexed Crimea after a pro-Russian leader was toppled in Kiev.

Moscow later backed pro-Russian separatists in eastern Ukraine in a conflict in which more than 10,000 people have been killed. Major fighting ended with a 2015 ceasefire but deadly exchanges of fire are still frequent.

Poroshenko urged Berlin to halt an undersea pipeline project that would allow Russia to supply more gas to Germany directly.

The Nord Stream 2 project is a potentially serious problem for Ukraine, which currently earns large transit fees from piping Russian gas to Europe and stands to lose out.